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300-410 Practice Question: An engineer configures IPv6 RA Guard on a switch…
An engineer configures IPv6 RA Guard on a switch port connected to a router running OSPFv3. Unexpectedly, OSPFv3 neighbor adjacencies fail to form on that link. Which is the most likely explanation?
⚠ Common exam trap
Cisco often tests the misconception that RA Guard only filters Router Advertisements, when in fact the device-role host option blocks all non-ND/DHCPv6 traffic, including OSPFv3 hellos.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
RA Guard with device-role host drops all IPv6 traffic except Neighbor Discovery and DHCP, including OSPFv3 hellos.
RA Guard with the device-role host command is designed to block all IPv6 traffic except Neighbor Discovery (ND) and DHCPv6 on the port. This includes OSPFv3 hellos, which use multicast address FF02::5. Since OSPFv3 relies on these hellos to form neighbor adjacencies, blocking them prevents adjacency establishment.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
RA Guard with device-role host drops all IPv6 traffic except Neighbor Discovery and DHCP, including OSPFv3 hellos.
Why this is correct
RA Guard host role restricts traffic to ND and DHCP only, blocking OSPFv3.
- ✗
OSPFv3 uses multicast address FF02::5 which is filtered by RA Guard by default.
Why it's wrong here
RA Guard does not filter OSPFv3 multicast unless configured to do so.
- ✗
RA Guard changes the MAC address of the router, causing OSPFv3 neighbor to be unreachable.
Why it's wrong here
RA Guard does not modify MAC addresses.
- ✗
The router must send Router Advertisements for OSPFv3 to work, and RA Guard blocks them.
Why it's wrong here
OSPFv3 does not require RAs.
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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